1. Field of the Invention
This invention relates to an improved heat exchange apparatus for transferring heat from a condensing vapor to an evaporating liquid. More particularly, this invention employs a novel heat transmitting membrane and configuration. One surface of the membrane retains a liquid which is heated to vaporization temperature by the latent heat of condensation transmitted from a vapor being condensed on an opposing surface of the membrane. The configuration employed permits packaging a large heat transfer area into a small volume.
The apparatus herein disclosed is applicable to desalinization of sea water, brine or brackish water. The U.S. Navy uses vapor compression type desalinization units aboard submarine and small craft where daily requirements for fresh water do not exceed 4000 gallons per day. Steam distillation plants in naval service utilize low pressure auxiliary exhaust steam, operate at less than atmospheric pressure and produce from 4000 to 50,000 gallons of fresh water per day. Extremely large, complex and expensive land based desalinization plants producing up to 250 million gallons of fresh water per day have been constructed for geographical areas deprived of fresh water.
A governmental study recently estimated that 3.6 billion gallons of hazardous wastes were injected into the earth by U.S. industry in 1981. Most of this was water contaminated with toxic chemicals. One estimate placed the number of injection well pumps between five and ten thousand, meaning the average injection well pumps around ten to twenty thousand gallons of hazardous wastes per day. Many of these wells are located in the vicinity of aquifiers providing domestic water. Water contaminated with toxic chemicals may be distilled to clarify the water and concentrate the toxic materials for reprocessing or disposal.
A great deal of water used in industrial operations is slightly contaminated and then discarded to the environment. For example, the rinse water from electro-plating operations, and the like, may constitute a water pollution problem due to heavy metal ions contained. If the rinse could be ecomomically distilled and reused, the pollution would be reduced and the distilled water would be preferable for reuse and valuable materials could be recovered from the residue.
The subject invention may also be used for certain other purposes not normally employing distillation, such as concentration of fruit and vegetable juices, manufacture of instant coffee or sugar and clarification of volatile solvents.
2. Description of the Prior Art
Improved heat transfer performance, heat transfer enhancement, augmentation or intensification has been attempted since J. P. Joules classic study of condensers in 1861. The number of publications concerning heat transfer enhancement have grown at an exponential rate since 1920. Nearly 500 U.S. patents related to enhancement technology have been issued. Most efforts have been directed to achieve the transfer of the maximum quantity of heat with the minimum expenditure of energy during the distillation process.
Enhancement technology has included passive techniques requiring no direct application of external power and active enhancement techniques which require external power. Passive enhancement techniques include treated heat transfer surfaces, rough surfaces, extended surfaces, swirl flow devices, coiled tubes and surface tension devices. Active enhancement techniques include mechanical aids, vibration of heat transfer surfaces or the fluids, electrostatic fields and flow inducement. Two or more of the techniques may be used to provide a "compound enhancement" greater than that of the individual techniques. The subject invention employs unique materials, surface treatment and configurations which may be generally classified as passive techniques.
Shell-and-tube heat exchangers are commonly used to exchange heat between boiling or condensing fluids and liquids. The plates or tubes are used to separate the fluids and constitute the physical barrier through which heat is transferred from one fluid to another. A variety of structured surfaces are applied to the interior and/or exterior of the tubes or surfaces of plates. The structured surfaces operate to spread liquid films over large surface areas and/or reduce the thickness of the fluid films.
The simplest and most universally understood type of apparatus used to distill water on a large scale is the vapor compression still. The vapor extracted from the feed water is heated to a higher temperature by substantially adiabatic compression. The vapor will therefore condense at a higher temperature than that at which evaporated, and as the vapor condenses, its latent heat of vaporization/condensation is thus recovered and recycled continuously. The net energy input, in the form of work of compression, is only that required to cause heat to flow from the condensing vapor to the evaporating liquid.
By conducting the distillation process in a rarified atmosphere or vacuum, the temperatures required can be lower than if performed at atmospheric pressure. The reduced temperatures required can reduce heat loss from the system and may avoid necessity to preheat the feed liquid or provide heat within the distillation apparatus.
Vapor compression stills operated at atmospheric or higher pressures or within a vacuum are common practice. Such stills operate with small thermal gradients across the heat transfer wall separating the evaporation (heated) side of the wall from the condensation (heating) side of the wall. Substantial effort has been applied to improve the flow of the heat energy across the wall. High thermal conductive materials and exceedingly thin wall sections have been employed. The fluid film on each side of the wall may be almost motionless, thus heat transfer through the film is very poor. The major part of the thermal resistance occurs in the fluid film rather than in the wall. Thicker fluid films are more resistant to heat transfer than thinner fluid films.
The velocity of fluid flow and amount of turbulence in the flow affect heat transfer. Increasing the velocity of fluid flow diminishes the thickness of the fluid film and thus increases heat transfer. Turbulent flow breaks up the fluid film effecting transposition within the flow and thus increases heat transfer. Although there are some disadvantages to excessive turbulence, most recent efforts have been to develop heat exchangers with a certain amount of turbulence so that fluid films may be kept to a minimum thickness.
J. B. Hammer in his U.S. Pat. No. 3,282,797 employs a thin evaporation plate structure with configured surfaces intended to improve the liquid spreading characteristics on the evaporation side. Hammer relies upon the surface tension of the liquid during essentially unobstructed gravity flow over a configured surface to form an exceedingly thin liquid film. The thinness of the film is primarily determined by the pressure gradient due to the surface tension of the liquid which may be augmented by the pressure gradient due to the gravitational force on the liquid.
The preferred embodiment of Hammer's structure is an evaporation plate inclined with the horizontal so the gravitationally induced pressure gradient augments, but does not nullify, the effect of the very particular configuration of the flow distributing surface. The present invention employs substantially vertical surfaces with an essentially random configuration of the flow distributing surface rather than the definitive channel distributing configuration taught by Hammer.
D. W. Elam in his U.S. Pat. No. 3,161,574 employs a thin resinous plastic, water-impermeable film as the heat transfer wall. The plastic film is not wetted by water and must be physically supported and shaped by a containing perforated screen or open mesh material. In addition to providing the required mechanical support to the film, the screen contributes to distribution of the liquid flow over the plastic film and ensures turbulence to the descending liquid flow, preventing formation of a stagnant water film.
Both Elam and the present invention employ thin plastic films or membranes as heat transfer mediums. However, Elam requires a structural support of his plastic film with a supporting and shaping screen; whereas the present invention contemplates a film which will maintain its structural integrety and develop the desired configuration upon imposition of a differential pressure across the film. Further, Elam relies upon rapid turbulent flow induced by a screen to enhance heat transfer, specifically disclaiming a layer of stagnant water, capillary action and a "wicked" evaporating surface as proposed by the present invention.
G. L. Henderson in his U.S. Pat. Nos. 3,414,483 and 3,586,090 employs a gravity induced high velocity to a flowing film of brine to enhance heat transfer to the brine. A free fall of the brine for about three feet over a smooth vertical heat transfer surface is used to reduce film thickness and promote turbulent flow. Henderson also teaches the collection of condensate in traps spaced vertically along the heat transfer wall to control film thickness of the condensed vapor.
In his U.S. Pat. No. 4,094,734 G. L. Henderson discloses a tubular evaporator in which a falling film of viscous brine or viscous solution is provided on the interior surfaces of vertically disposed heat transfer tubes by a pump and distribution system. The distribution system has constant cross-sectional areas to maintain substantially constant velocity of recirculated viscous solutions. Conical distributors are employed to spread and deposit the viscous solutions on the interior surfaces of the heat transfer tubes. Henderson also teaches the collection and removal of condensate in traps hellically disposed about the exterior of the heat transfer tubes.
The free falling flow used by Henderson to achieve a thin film of distilland or viscous solution is not anticipatory of the present invention. Further, the vertical distances required by Henderson necessitate the vertically distributed condensate traps which are not required or anticipated for the present invention.
F. J. Castle et al, in their U.S. Pat. No. 2,530,376 present an apparatus for vacuum distillation employing a readily detachable mesh fabric to distribute a film of distilland to a heated evaporation surface. The mesh fabric is used to increase the sojourn of the distilland, prevent channeling and be readily removeable for cleaning of residue or replacement.
The invention of Castle et al, most closely resembles the present invention in the use of a mesh fabric to distribute the distilland over a heated evaporation surface. However, the basic concept and objective is not anticipatory of that disclosed by the present invention. The present invention does not require a heat source within the apparatus and vaporization of the distilland is effected upon the same structure upon which condensation takes place, not across a spaced relationship as taught by Castel et al.
To summarize:
Distillation processes in current use may be performed in a rarified or vacuum environment. Thin films or membranes have been used as heat transfer mediums. Contorted and/or porous surfaces have been used to distribute distilland, induce flow as a thin film and promote turbulence. Present distillation processes usually require preheating the distilland or addition of heat or mechanical energy during the distillation process. Relatively expensive materials and equipment are currently used for distillation apparatuses which are not particularly efficient.
The theoretical energy required to extract one thousand gallons of distilled water from two to three times that amount of sea water is around four kilowatt hours. Even large and well designed sea water distillation plants in current use require at least twenty times the theoretical energy, i.e., are five percent efficient or less. Energy requirements are minimal where distillation can be effected at low temperatures.
The greater the heat transfer area, the less the temperature and pressure difference requires to cause a given quantity of heat flow. Very large heat transfer areas are required to produce substantial quantities of condensate at the lower temperatures. Given unlimited heat transfer area, the ideal energy required to distill water with only a trace of dissolved matter would approach zero. Clearly, there is a trade-off between energy cost and the capital cost of surface area made available for heat transfer. There are no methods or equipment presently available which can provide the large heat transfer areas to efficiently distill large quantities of sea water at low temperatures at acceptable capital and operational costs.